A cellular communication system projects a number of cells onto the Earth at diverse locations. A frequency spectrum is allocated in frequency, time, coding or a combination of these to the cells so that communications occurring in nearby cells use different channels to minimize the chances of interference. Communications taking place in cells located far apart may use the same channels, and the large distance between communications in common channels prevents interference. Over a large pattern of cells, a frequency spectrum is reused as much as possible by distributing common channels over the entire pattern so that only far apart cells reuse the same spectrum. An efficient use of spectrum results with no interference among the different communications.
One problem which cellular communication systems address is the handing-off of communications between cells. Relative movement between end users and cells causes the end users and the communication links directed thereto to move between cells. To permit continuous communications in an ongoing call, the system must "handoff" the call when the individual subscriber unit crosses a cell boundary. If a call is not handed-off to a new cell upon leaving the old cell, the call will eventually be lost or dropped because the strength of the signals significantly decreases so that the individual subscriber unit cannot receive the unit's transmissions, or visa versa.
The conventional handoff technique may work adequately when the distances between individual subscriber units and system transceivers are relatively small, when the speeds of movement between cells and subscriber units are slow, or when handoffs are relatively evenly distributed in time. Such conditions are present for conventional terrestrial cellular systems in which cells do not move with respect to the Earth and in which subscriber units move between cells according to conventional modes of transportation. On the other hand, when system radio equipment is located on satellites orbiting the Earth at high rates of speed, these conditions are not present and conventional handoff techniques are inadequate.
Some advanced handoff techniques use a predetermined sequence or schedule for determining which channel of what cell to handoff communication. The predetermined schedule for a particular ISU is derived by first providing to a central controller a link with the system and its position on the Earth. Next, the central controller projects the satellite orbital motion against the ISU position to derive a schedule of handoffs for the ISU. Finally, the predetermined schedule of handoffs is sent to the ISU which attempts to execute the handoffs as directed by the schedule.
Scheduled handoffs depends on an accurate location for every subscriber. Generating and maintaining the schedules for all of the ISU's currently active in the network places a major processing burden on gateways in the network. Moreover, the fixed scheduling is susceptible to failures based on instantaneous local conditions or circumstances. Failures can be caused by factors such as fading or subscriber motion. This leads to a large number of error checks and recovery processes.
Accordingly, there is a significant need for a cell hand-off system which permits each of the ISU's to determine when a handoff is necessary and to select a candidate cell that will receive its communications, rather than imposing a single universal handoff process for all ISU's.